It is known that an antenna with a plurality of resonant helical elements arranged around a common axis can be made to exhibit a dome-shaped spatial response pattern which is particularly useful for receiving signals from satellites. Such an antenna is disclosed in "Multielement, Fractional Turn Helices" by C. C. Kilgus in IEEE Transactions on Antennas and Propagation, July 1968, pages 499 and 500. This paper teaches, in particular, that a quadrifilar helix antenna can exhibit a cardioid characteristic in an axial plane and be sensitive to circularly polarised emissions. The antenna comprises two bifilar helices arranged in phase quadrature and coupled to an axially located coaxial feeder via a split tube balun for impedance matching. While antennas based on this prior design are widely used because of the particular response pattern, they have the disadvantages that they are extremely difficult to adjust in order to achieve phase quadrature and impedance matching, due to their sensitivity to small variations in element length and other variables, and that the split tube balun is difficult to construct. As a result, their manufacture is a very skilled and expensive process.
It is an object of this invention to provide an antenna which achieves similar performance to those of the prior art at lower cost.